Caveat: I have no medical nor physio qualifications.
Last year I was icing my knees after every run. More to prevent problems than anything big, but my knees did ache almost constantly. It was always in the back of my mind how much longer I could run as my preferred exercise, which depressed me.
I started reading around in response to several unconnected sources that all pointed to the same thing - that knee and joint damage should not be a normal part of running and that by changing how you run, you change the stress on your knees. There's lots out there about POSE and barefoot running and just plain forefoot strike running and it essentially seems to be grounded in the same idea - that heel strike running, promoted by shoes, causes most of the problems that runners have. Our feet seem to be engineered perfectly to absorb shocks when we use them right (and also aren't carrying around an extra 20 or 100 pounds, of course!).
I don't follow any particular running method and I still run in shoes (I have tender feet). But about 8 months ago I started to incorporate running on the balls of my feet into my runs. I gradually increased the proportion of forefoot and "regular" strike as I could. In the beginning, it did feel weird. Very weird.
I felt like I should have a basket of flowers hanging over one arm as I pranced around stupidly. But that passes (and it helps to know that from the outside it doesn't look anything like as peculiar as it feels). And the muscles you use in your feet and calves with a forefoot strike are different, so I had sore muscles and blisters all over again. But now I can do a whole 5 or 10k running this way and it feels marvelous. The only time I have used an ice pack on my knees in the last 6 months was a few weeks ago right after I went to my first karate class and I had to sit on my knees on a hard wooden floor.
For me it boiled down to my belief that the human body is designed pretty well for things it's supposed to do, and that running is one of them. Just look at kids - they run everywhere. I don't buy that something that almost all children do so instinctively and frequently isn't natural. And my knee pains have disappeared with the new way of running, along with many of my fears that I was going to have to find something else to do.
Oh, my runs are 5-10km each, 3-4 days a week (I'm not very fast). So pretty much within what Jeff said (except that 10km=6.25miles). I would like to run a half marathon in the next year now that I've got a number of 10km races under my belt. But that's probably my limit.
Just my thoughts. Good on you for the races, and keep at it, at whatever level you're comfortable.